Calculate Soakaway Size: A Step-by-Step UK Guide
Table of Contents
- What Is a Soakaway and Why Size Matters
- How to Calculate Soakaway Size: The BRE Digest 365 Approach
- Soil Percolation Testing for Infiltration Rate
- Calculating Storage Volume and Design Rainfall Depth
- Soakaway Regulations UK and Building Control Compliance
- Soakaway Crates: Installation and Material Considerations
- Maintenance and Longevity of Your Soakaway System
- Troubleshooting Failed Soakaways: Common Issues and Solutions
Last Updated: July 28, 2026
What Is a Soakaway and Why Size Matters
A soakaway is an underground structure designed to collect and slowly infiltrate surface water into the soil, reducing runoff and managing stormwater on residential and commercial properties. Understanding how to calculate soakaway size is essential for anyone installing a drainage system that complies with UK Building Regulations and environmental standards. The most common mistake is underestimating the volume required.
An undersized soakaway floods during heavy rainfall and can cause water to back up into your property or neighbouring land. Conversely, an oversized soakaway wastes money and space without adding meaningful benefit. The calculation requires accurate site data and understanding of local soil conditions.
The real issue isn’t the maths, it’s getting the soil [infiltration rate](/7/infiltration-in-suds-when-it-works-when-it-fails-and-what-evidence-you-need/) correct. Most failed soakaways happen because the percolation test was rushed or the soil conditions changed between testing and installation.
How to Calculate Soakaway Size: The BRE Digest 365 Approach
The industry standard for soakaway design in the UK is BRE Digest 365, published by the Building Research Establishment. This methodology combines rainfall intensity, roof catchment area, soil infiltration rate, and storm duration into a single formula that determines the storage volume your soakaway must provide. Building Regulations Part H requires that any surface water drainage system, including soakaways, be designed using this or an equivalent approved method.
The calculation formula is:
Storage Volume (m³) = Rainfall Depth (mm) × Catchment Area (m²) ÷ 1000 ÷ Infiltration Rate Multiplier
Precision at the data-gathering stage prevents costly redesigns later. Each variable, rainfall intensity, catchment area, and soil percolation rate, requires specific measurement or research.
Step 1: Measure Your Roof Catchment Area
Your catchment area is the total surface from which water will drain into the soakaway. For most domestic properties, this is the roof area, though in commercial applications it might include paved surfaces, car parks, or other impermeable surfaces.
To calculate roof catchment area, measure the horizontal projection of your roof, not the sloped surface area. Multiply the length by the width of the building footprint. If your roof has a complex shape, break it into rectangles and add them together. For a simple rectangular house measuring 10 metres by 8 metres, the catchment area is 80 m².
The runoff coefficient is a multiplier that accounts for how much of the rainfall actually reaches the soakaway. For pitched roofs with guttering, the runoff coefficient is typically 1.0 (meaning 100% of rainfall reaches the system). For paved surfaces or gravel, it may be 0.8 or lower, depending on permeability. Building Regulations Part H guidance specifies these coefficients.
Step 2: Determine Rainfall Intensity and Storm Duration
Rainfall intensity varies across the UK, and BRE Digest 365 uses design rainfall data based on location and storm frequency. The standard approach is to use a 1-in-100 year storm event with a duration of 30 minutes for most domestic applications.
Your design rainfall depth depends on your postcode. The Environment Agency and UK Climate Projections (UKCP) provide rainfall intensity maps and data tables by region. For example, properties in the south-east might experience 20-25 mm of rainfall in a 30-minute storm, whilst properties in Scotland or northern regions might see 15-20 mm under the same conditions.
Design rainfall depth is location-specific and non-negotiable. Using the wrong value is one of the fastest ways to end up with an undersized soakaway that fails in the first heavy rain.
Step 3: Apply the Runoff Coefficient
The runoff coefficient accounts for losses during rainfall. Not all water that falls on a roof reaches the soakaway; some evaporates and some soaks into the soil during the rainfall event. For a pitched roof with standard guttering, the runoff coefficient is 1.0. For permeable surfaces, it’s lower.
Multiply your catchment area by the runoff coefficient to get your effective catchment area. For a 100 m² roof with a coefficient of 1.0, the effective area is 100 m². For a paved surface of 100 m² with a coefficient of 0.8, the effective area is 80 m².
Soil Percolation Testing for Infiltration Rate
The infiltration rate, how quickly water drains through your soil, is the single most critical input to the calculation. Get this wrong, and your soakaway either works perfectly or floods your garden. Infiltration rate is measured in millimetres per hour (mm/h). Sandy soils infiltrate quickly (50+ mm/h), clay soils infiltrate slowly (5-10 mm/h), and chalk or limestone can vary widely. You cannot estimate this from looking at the soil. You must conduct a percolation test on site.
How to Conduct a Percolation Test
A percolation test is a simple field test that measures how quickly water drains through your soil. The test takes about 2-3 hours.
What you need:
- A spade or auger to dig a hole
- A measuring ruler or tape
- A bucket or hose to supply water
- A stopwatch or timer
- A notebook to record results
Steps to conduct the test:
- Dig a hole at the proposed soakaway location, roughly 300 mm deep and 300 mm wide.
- Fill the hole with water and allow it to drain completely. This pre-wets the soil and removes the effect of dry soil absorbing water rapidly.
- Refill the hole to a known depth (typically 100 mm marked with a ruler).
- Measure how long it takes for the water level to drop by 25 mm. Record this time in minutes.
- Repeat at least twice more, refilling the hole each time.
- Calculate the average time for the 25 mm drop.
The infiltration rate in mm/h is calculated as: (25 mm ÷ average time in minutes) × 60 = mm/h
For example, if water drops 25 mm in 10 minutes on average, the infiltration rate is (25 ÷ 10) × 60 = 150 mm/h. This is a sandy soil with good drainage.
If your test shows an infiltration rate below 1 mm/h, the soil is too impermeable for a soakaway. You’ll need to use an alternative surface water drainage method, such as a drainage pipe to a watercourse or public sewer.
Interpreting Percolation Test Results
BRE Digest 365 groups infiltration rates into categories:
- Above 50 mm/h: Very permeable soil (sand, gravel). Soakaway will be small.
- 20-50 mm/h: Permeable soil (sandy loam). Standard design applies.
- 10-20 mm/h: Moderately permeable soil (loam). Larger soakaway required.
- 5-10 mm/h: Low permeability (clay loam). Very large soakaway needed.
- Below 1 mm/h: Impermeable soil (heavy clay). Soakaway not suitable.
Conduct the percolation test at the exact location where the soakaway will be installed. Soil properties vary across even a small garden, and testing 5 metres away from the actual site could give misleading results.
Calculating Storage Volume and Design Rainfall Depth
Now you have all the inputs: catchment area, rainfall depth, runoff coefficient, and infiltration rate. The final step is calculating the storage volume your soakaway must provide.
Formula:
Storage Volume (m³) = (Rainfall Depth in mm × Catchment Area in m² × Runoff Coefficient) ÷ (Infiltration Rate in mm/h × Storm Duration in hours × 1000)
Let’s work through a practical example:
- Catchment area: 100 m² (roof)
- Design rainfall depth: 20 mm (south-east England, 1-in-100 year, 30 minutes)
- Runoff coefficient: 1.0 (pitched roof)
- Infiltration rate: 25 mm/h (sandy soil from percolation test)
- Storm duration: 0.5 hours (30 minutes)
Calculation:
(20 × 100 × 1.0) ÷ (25 × 0.5 × 1000) = 2000 ÷ 12,500 = 0.16 m³
This property needs a soakaway with 0.16 m³ of storage volume. A standard soakaway crate (typically 0.5-1 m³ capacity) would be adequate.
Now consider a different scenario with clay soil:
- Same catchment area and rainfall depth
- Infiltration rate: 5 mm/h (clay soil)
- All other values the same
Calculation:
(20 × 100 × 1.0) ÷ (5 × 0.5 × 1000) = 2000 ÷ 2,500 = 0.8 m³
The same property now needs 0.8 m³ of storage because the soil drains more slowly. You’d need multiple soakaway crates or a larger excavated pit.
Soakaway Regulations UK and Building Control Compliance
Every soakaway installed in England must comply with Building Regulations Part H (Drainage and Waste Disposal). Scotland, Wales, and Northern Ireland have equivalent standards. Failure to comply can result in enforcement action, difficulty selling the property, or insurance claims if the system fails.
Building Regulations Part H Requirements
Part H requires that surface water drainage systems be designed to handle a 1-in-100 year rainfall event with an appropriate safety margin. The design must be based on site-specific data (percolation testing) and follow an approved methodology such as BRE Digest 365.
Your soakaway must be:
- Designed using BRE Digest 365 or equivalent approved method
- Based on a percolation test conducted at the installation site
- Sized to handle the design rainfall for your location
- Sited at least 5 metres from any building (including your own)
- Sited at least 2.5 metres from any boundary
- Installed with appropriate backfill and geotextile membrane to prevent clogging
- Maintained and inspected regularly to ensure continued function
Building Control will review your drainage design before work begins. If the design doesn’t meet Part H, they’ll ask for revisions.
Siting Distance from Buildings and Boundaries
The 5-metre minimum distance from buildings exists because soakaways can affect ground stability and cause subsidence if water infiltrates too close to foundations. This distance applies to your own building and any neighbouring structures. If your property is close to a boundary, the 2.5-metre distance from the boundary may limit where you can site the soakaway.
For corner plots or small gardens, siting can be challenging. In these cases, you may need to use a different drainage solution, such as a pipe to a public sewer or watercourse.
Soakaway Crates: Installation and Material Considerations
Modern soakaways typically use plastic crates rather than traditional gravel pits. Soakaway crates are modular, stackable, and provide consistent void ratio (the proportion of empty space that stores water). They’re easier to install, more predictable in performance, and less prone to clogging than loose aggregate.
A typical soakaway crate measures roughly 1 metre by 1 metre by 1 metre and provides approximately 0.5-0.6 m³ of storage volume. To achieve your calculated storage volume, you install multiple crates stacked vertically or arranged side by side.
Void Ratio and Geotextile Membrane Selection
Void ratio is the percentage of the crate that’s empty space (available for water storage). Standard plastic soakaway crates have a void ratio of 85-95%, meaning most of the volume is usable storage. This is much higher than gravel-filled pits, which typically achieve 40-50% void ratio.
Wrapping the soakaway in a geotextile membrane prevents fine soil particles from washing into the crate and clogging it over time. The membrane is a permeable fabric that allows water to pass through but blocks soil. Install the membrane around the sides and base of the soakaway, leaving the top open or loosely covered during installation so water can enter from above.
Backfill the excavated pit with a layer of coarse gravel or recycled aggregate around the crate. This provides a transition zone between the fine soil and the soakaway, reducing the risk of clogging. The gravel layer should be at least 150 mm on all sides.
Maintenance and Longevity of Your Soakaway System
A properly designed and installed soakaway can function for 20-30 years or longer. However, maintenance is essential. Without it, the system clogs within 5-10 years and stops working.
Preventing clogging:
Install a silt trap or sediment chamber upstream of the soakaway. This simple chamber collects fine particles, leaves, and debris before they reach the soakaway. Clean the trap out twice a year, particularly in autumn when leaf fall is heavy. This single step extends soakaway life by 10+ years.
Ensure guttering and downpipes are kept clear. Blocked gutters cause water to overflow and bypass the soakaway entirely.
Inspection and monitoring:
Visually inspect the soakaway area twice yearly. If water is pooling or the ground is waterlogged even days after heavy rain, the soakaway may be clogging. Early detection allows you to clean the silt trap before the problem becomes severe.
A silt trap adds years to the system’s life. It’s one of the most cost-effective investments you can make in your [drainage system](/79/what-property-developers-need-to-know-about-sustainable-drainage/).
Troubleshooting Failed Soakaways: Common Issues and Solutions
Even well-designed soakaways fail sometimes. Understanding the causes helps you fix the problem correctly rather than simply replacing the system.
Symptom: Water pools above the soakaway immediately after rain
This indicates the soakaway is either clogged or undersized. If the system was installed recently, undersizing is likely. If the system has been working for years and suddenly fails, clogging is the cause.
Fix: Excavate the soakaway and inspect. If clogged, remove the top layer of soil and geotextile, clean out accumulated silt, and replace the geotextile. If undersized, add additional crates or replace with a larger system.
Symptom: Water appears in the basement or crawl space during heavy rain
The soakaway is failing to handle the design rainfall, and water is backing up into the building. This is a serious issue requiring immediate action.
Fix: First, confirm the soakaway is actually receiving water (check that gutters and downpipes are clear). If water is reaching the soakaway but still backing up, the system is undersized or clogged. Temporary solution: install a pump to divert water away from the building. Permanent solution: upgrade the soakaway or switch to a different drainage method.
Symptom: Neighbouring property is waterlogged or flooded
Your soakaway is discharging water onto neighbouring land, or the system is so undersized that water is overflowing and running off-site. This creates legal liability and must be fixed immediately.
Fix: Engage a drainage engineer to assess the system and redesign if necessary. You may need to install a larger soakaway, add a second soakaway, or use an alternative drainage method such as a pipe to a public sewer or watercourse.
Symptom: Soil is subsiding or cracking near the soakaway
This indicates water is infiltrating too close to foundations or causing ground instability. It’s a structural issue requiring professional assessment.
Fix: Stop using the soakaway immediately and contact a structural engineer. The soakaway may be too close to a building, or the soil may not be suitable for infiltration-based drainage. You’ll likely need to decommission the soakaway and install a different system.
| Issue | Cause | Solution |
|---|---|---|
| Water pools above soakaway | Clogged or undersized | Clean silt trap; add crates or redesign |
| Basement flooding during rain | System failing to handle design rainfall | Upgrade soakaway or switch drainage method |
| Neighbouring property waterlogged | Undersized or overflowing system | Enlarge soakaway or use alternative method |
| Soil subsidence near soakaway | Water infiltrating too close to building | Decommission and install different system |
Calculating soakaway size correctly requires accurate data, methodical work, and attention to detail. The investment in a proper percolation test and careful design saves money and prevents failures that are far more expensive to fix. Get in touch with The Drainage Designers (Woodvale consulting Ltd) if you’re designing a system for a new build, extension, or renovation. Our team conducts site visits, runs percolation tests, and produces designs that secure Building Control approval first time, ensuring your project stays on schedule and your drainage system works reliably for decades.
Frequently Asked Questions
How is soakaway size calculated in the UK?
Soakaway size is calculated using BRE Digest 365 methodology, which combines roof catchment area, rainfall intensity, design rainfall depth, and soil infiltration rate. Measure your roof area in square metres, determine the design rainfall depth for your region (typically 5mm for a 1-in-100-year storm), and divide by your soil’s infiltration rate (measured via percolation test). This gives you the required storage volume in cubic metres, which determines the physical dimensions needed for your soakaway crate or excavated pit.
What is a percolation test and why is it needed for a soakaway?
A percolation test measures how quickly water infiltrates into your soil, expressed as millimetres per hour. It is essential because infiltration rate directly determines how large your soakaway must be. Soil with high infiltration rates (sandy, gravelly soils) requires smaller soakaways; clay soils with low infiltration rates require much larger storage volumes. Building Regulations Part H requires evidence of soil percolation testing before soakaway design can be approved by building control.
What are the UK building regulations for soakaways?
Building Regulations Part H governs surface water drainage, including soakaways. Key requirements include: percolation testing must demonstrate the soil can accept water; soakaways must be sited at least 5 metres from buildings and 2.5 metres from boundaries; they must not be placed over contaminated land or within 10 metres of a water source; and the design must comply with Sustainable Drainage Systems (SuDS) principles. Local authorities may impose stricter requirements, so check with your local building control office.
How many soakaway crates do I need for a typical domestic roof?
The number of soakaway crates depends on your calculated storage volume and the void ratio of the crates you select. For example, if your calculation shows you need 10 cubic metres of storage and each crate provides 0.65 cubic metres (typical void ratio), you would need approximately 15-16 crates. However, this varies significantly based on rainfall intensity for your postcode, roof catchment area, and soil infiltration rate. Use the BRE Digest 365 formula with your site-specific percolation test results for an accurate figure.
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